The Coriolis mass flowmeters with straight tube geometry, which are of interest here, are optimal in flow terms and are employed mainly in process engineering plants, in order to measure mass throughflows through a pipeline. For this purpose, the meter excites the measuring tube, through which fluid flows, into periodic oscillation. The influence of the fluid flow on the oscillation behavior is measured at least two locations on the measuring tube. The mass throughflow can be determined from the phase difference of the measurement signals at the measurement locations.
A generic Coriolis mass flowmeter is known from DE 103 51 312 A1. This consists of a straight measuring tube which oscillates in coupled flexion and torsion modes and the oscillation behavior of which is detected by sensor technology for the purpose of subsequent signal evaluation. The straight measuring tube has connected to it mechanically a mounted part which is designed rotationally symmetrically with respect to a rotational symmetry axis and which can be set in torsional oscillations of the same frequency as, but opposite phase position to, the torsional oscillation modes of the measuring tube. The mounted part is a multipart body which may consist of hollow profile rails and balancing elements.
The force action of the fluid on the measuring tube wall on account of the flow is very low, as compared with other forces which arise. So that the measurement effect can be distinguished from background and interference, the construction and symmetry of the meter must satisfy stringent requirements. In particular, however, the meter must be decoupled as completely as possible in oscillation terms from its surroundings, in particular the pipeline. Such decoupling, which is also designated as balancing, is achieved here by means of the mounted parts.
A further Coriolis flowmeter, which is stabilized by means of a compensation cylinder surrounding the measuring tube, may be gathered from EP 0 985 913 A1. The compensation cylinder is connected to the measuring tube in a way in which axial relative movements are ruled out. As a result, expansions or stresses which arise due to the straight design of the measuring tube are compensated. These expansions or stresses which arise in the event of temperature differences would otherwise impair the measurement accuracy. In an extreme instance, stresses induced thermally in this way may even lead to mechanical damage, to be precise to stress cracks, on the measuring tube.
Furthermore, from the general prior art in the field of Coriolis mass flowmeters with a straight measuring tube, it is known to produce measuring tubes from a corrosion-resistant metal, preferably from titanium or titanium alloys. Titanium and its alloys, because of their mechanical properties, to be precise a relatively low thermal expansion and low rigidity (modulus of elasticity), may be considered for a wide range of temperatures in use. Moreover, titanium is resistant to a multiplicity of corrosive media.
To implement the Coriolis measurement principle, further mounted parts, such as drive and balancing elements and end plates, are provided on the titanium measuring tube. In addition, there are stabilizing elements which are arranged between the end plates, are connected to the end plates and are therefore coupled to the measuring tube via the end plates.
It is conceivable, in principle, that the mounted parts and stabilizing elements also consist, like the measuring tube, of titanium or a titanium alloy. However, titanium is less suitable for use in the stabilizing element and, in practice, is also not used for this purpose, since a material having a higher density and consequently, with a comparable volume, having a higher mass is required for this application. In generic Coriolis mass flowmeters, therefore, the mounted parts and the stabilizing elements consist of a metal other than that of the measuring tube.
The mounted parts and the stabilizing elements are usually manufactured from steel which has a coefficient of thermal expansion other than that of the titanium measuring tube and are connected to the measuring tube via connection techniques, such as brazing or welding. The same connection techniques are also employed for connecting the stabilizing elements to the end plates.
Brazing, in particular, requires various brazing alloys in order to implement a high-quality connection between the titanium of the measuring tube and the metal of the other mounted parts.
During the attachment, taking place while the meter is being manufactured, of stabilizing elements, running parallel to the measuring tube, between the end plates, in particular by brazing or welding, however, the different coefficient of expansion of the titanium measuring tube and of the stabilizing elements consisting of steel proves to be a disadvantage. Owing to the different expansion during heating to brazing temperature or welding temperature and to contraction during cooling, stresses are introduced into the structure and lead to warpings and distortions during brazing or welding. In order as far as possible to prevent this, usually a plurality of brazing or welding steps are carried out in a defined order, although this correspondingly increases the outlay in production terms.